Preventing Voids When Pad Printing on Textures

You might consider extending pad dwell time on the surface to give the pad more time to force the air out of the recesses or allow time for ink to flow into the recesses. However, once the air is trapped, it's trapped. Multiple hits of the pad will not solve the problem, since the air will be trapped the same way each cycle. Also, pad-printing ink does not flow. The addition of a flow agent simply allows the pad to "slide," which can degrade print definition or sharpness.

Pad printing is based on the transfer of a tacky ink. After the pad picks the ink out of the plate, the ink becomes semi-dry, and no additive will force it to flow. The only way to get full ink release is for the ink to make complete contact with the substrate, and only by using a hard pad can you ensure this contact.

The ultimate solution is for the part designer to consider printing requirements before specifying the texture. If textured areas will be printed, the designer should select a positive texture, which will look the same as a negative texture but will not cause printing problems.

Kaverman: Textures vary in their individual depth, the degree of draft (angle) of their side walls, and in the frequency of peaks and valleys for a given surface area. Typically, the deeper the texture, the greater the angle of the side walls to allow the part to release from the mold. Depending on these variables, some textures are more difficult to successfully print than others, and others are simply impossible to completely cover.

To find a correlation between these variables and a part's "printability," I printed a series of approximately sixty plaques with various textures, all of which were molded out of ABS plastic. I used an automotive approved, two-component white ink (thinned 15% by weight); a steel cliche with an etch depth of 0.001 in., and a 60-durometer (Shore scale A) transfer pad. Each plaque was single printed at one end, double printed on the other, then allowed to dry per the ink manufacturer's recommendations.

I then visually inspected each print for coverage. Acceptance or non-acceptance of the print was determined by the presence of any visible defect or void resulting from insufficient coverage of the texture.